The present invention relates to electrodeless high-intensity-discharge (HID) lamps and, more particularly, to a novel inverted excitation coil for initiating and maintaining a plasma arc discharge within the arc tube of the electrodeless HID lamp.
It is now well known to provide a toroidal light-emitting plasma within the envelope of a HID lamp. The induction arc plasma depends upon a solenoidal, divergence-free electric field for its maintenance; the field is created by the changing magnetic field of an excitation coil, which is typically in the form of a solenoid. It is necessary to develop a very high electric field gradient across the arc tube to start the plasma discharge; it is difficult to develop a sufficiently high electric field gradient, especially in the associated excitation coil, because the coil current may be prohibitively high, even if it is provided only on a pulse basis. Further, providing a very high electric field gradient may be impossible because the necessary field-per-turn of the excitation coil may exceed the turn-to-turn electrical breakdown rating of that coil. Thus, it is difficult to provide some means for starting induction-driven HID lamps, and it is also difficult to provide for hot restarting of the same type of lamp. While the use of a single spiral starting aid is described and claimed in co-pending allowed application Ser. No. 226,584, filed August 1988, assigned to the assignee of the present application and incorporated here in its entirety by reference, the use of even one additional structure, within a HID lamp, has negative cost and manufacturing impact. It is therefore not only highly desirable to provide some means for starting the HID lamp plasma discharge, but also to do so by means of some special configuration of the excitation coil, so that at least one additional member, utilized only for the starting operation, need not be provided.